(i) Field of the Invention
The present invention relates to an apparatus for predicting the life of a tool which detects work load values of a machine tool and indicates a tool life, when a remarkable change of the work load values is observed.
(ii) Description of the Prior Art
Heretofore, a tool life has been predicted by the use of a working term or the number of working times as a parameter representing the tool life on the basis of the experience of an operator. That is, a predetermined working term or a predetermined number of working times has been regarded as the tool life. FIG. 9 shows numbers of holes formed by some drills having one lot number until the drills have been broken. As is apparent from FIG. 9, even drills having the same lot number have different tool lives, and therefore it is difficult to predict the tool life.
According to another method, increase in the work load values before the breakage of the tool is detected, and the tool is then stopped by an alarm output of a work load monitor by which the above-mentioned increase in the work load values has been detected. Afterward, the condition of the tool is estimated by an operator to decide the timing of the tool exchange. Also in this case, however, the tool life depends upon the experience of the operator, and so the tool has often been broken. This is for the following reason: when the alarm is first output, it cannot be foreseen how long working can be continued until the tool is broken, and eventually the tool is utilized to its limit.
FlG. 7 is a block diagram illustrating the constitution of a conventional work load monitor, and FIGS. 8A and 8B show its signal processing method. The work load monitor has an A/D conversion section 1 in which the work load values (LS, see FIG. 8A) detected and then output in the form of signals in the machine tool are converted into digital data at a constant sampling period. The A/D conversion section 1 is connected to an averaging processing section 5 and a monitoring start deciding section 2. In the averaging processing section 5, data of the work load values of a number set in a sampling times setting section 4 are averaged. Furthermore, the monitoring start deciding section 2 outputs a monitoring start signal to the averaging processing section 5, when the work load values have reached a predetermined level.
The averaging processing section 5 outputs the obtained mean value as a mean signal (MS). The averaging processing section 5 is connected to a comparing section 13. This comparing section 13 compares the mean signal (MS) with an abnormal signal decision value (LIM) set by an abnormal signal decision value setting section 12 (see FIG. 8B), and it outputs an abnormal signal, when the data of the averaged work load values are in excess of the abnormal signal decision value (LIM).
To the comparing section 13, a timer section 15 is connected. This timer section 15 outputs an alarm (AL) in order to report that the work load values have remarkably changed, when a period (t, see FIG. 8B) during which the abnormal signals are continuously input is in excess of a period set by a timer value setting section 14.
Next, operation will be described. The work load values (LS) detected In the machine tool are converted into the digital data at a constant sampling period in the A/D converting section 1 and then averaged in the averaging processing section 5. This averaging can be achieved by calculating a mean value of the data of the work load values of the member set in a sampling times setting section 4. When this mean value signal (MS) is fed to the comparing section 13, the mean value signal (MS) is compared, in the comparing section 13, with the abnormal signal decision value (LIM) set by the abnormal signal decision value setting section 12. If it is judged as a result of this comparison that the data of the averaged work load values are in excess of the abnormal signal decision value (LIM), the comparing section 13 outputs an abnormal signal to the timer section 15. When a period (t) during which the abnormal signals are continuously input into the timer section 15 is in excess of a period set by the timer value setting section 14, the timer section 15 judges that the work load values have remarkably changed, and it outputs an alarm (AL).
The conventional work load monitor is constituted as described above, and so only the remarkable change of the work load values is reported to an operator. Therefore, the experience of the operator is necessary for the foresight of the tool life, and skill is required. That is, there has been the problem that an unskilled operator cannot predict tool exchange timing.